1. Field of the Invention
This invention relates to an artificial larynx and more particularly to an electrically-powered artificial larynx having pitch change capability.
2. Description of the Prior Art
Persons who have been deprived of the use of their vocal chords or larynx through paralysis or surgery for example, frequently employ a mechanical or electro-mechanical speech aid commonly known as an artificial larynx. The basic operating principal of such a device is to generate a tone having a fundamental frequency in the speech range of the average human voice and to introduce the tone into one of the resonant speech cavities of the user, such as the mouth or pharnyx. To speak, the user modulates the artificially-generated tone by varying the shape of the resonant speech cavities comprising the pharnyx, mouth and nose and by making the usual constrictions of the tongue, teeth and lips so as to articulate the modulated tone as human speech.
One well-known type of artificial larynx employs a tracheal tube with one end inserted into a surgically made aperture in the users throat which terminates at the users trachea. The force of accelation through the tube is applied to a reed which vibrates to produce the required tone. The sound from the vibrating reed is introduced into the mouth or nasal cavity by means of a second tube.
Another such device employs an electro-acoustic transducer which is mounted in the mouth of the user by securing it to a denture. Power from batteries is transmitted to provide an oscillatory signal which is applied to the transducer by means of a wire entering the user's mouth.
Still another type of artificial larynx employs an electro-mechanical vibrator adapted to be secured to or held against the outside of the users throat so as to induce a tone into the region of the pharnyx. The generated sound is transmitted through the tissues of the throat and into the mouth where it is articulated into words and phrases.
Still another type includes a sound generator which is coupled to a plastic tube or the like to transmit the generated sound into a person's mouth through the lips which are held open such that the mouth can then be used to articulate the generated sound into speech.
Each of the devices of the prior art is deficient in one or more important aspects. For example, none of the prior art devices includes a satisfactory system for controlling pitch. One device employs an arrangement designed to control pitch by varying the point of resonance in a mechanically resonant vibrator. The principal, although sound, cannot be applied effectively within the necessary limits of size and complexity. The instrumentation is therefore a compromise and the resulting changes in pitch lack the distinguishing characteristics of the human voice.
Additionally, most of the devices of the prior art lack a means for generating the basic tone within a spectrum that approximates the range of normal human speech and, accordingly, the sounds produced are defective in both tonal quality and intelligibility. Many of the systems of the prior art emply power systems requiring heavy duty batteries which must be carried by the user or fixed to his person or clothing. Such arrangements are both awkward and obstrusive and are therefore embarrassing to the user.
One of the devices of the prior art provides pitch control by providing a variable resistance in a relaxation oscillator circuit which generates the pulses to drive the transducer. This system requires operator dexterity in that the rate, the amount and the direction of pitch change must all be manually controlled as the operator increases and decreases the oscillator resistance. Use has shown that few persons are able to manipulate the rate of change of the pitch in such a manner so as to produce a pleasing, human-like quality of speech. Rather, a buzzing mechanical or computer-like sound is often produced and the manual manipulation required to control the rate of pitch change often leads to rapid increases or decreases in pitch or, in the alternative, to extremely slow or jerky, non-human sounding pitch changes.
These drawbacks of the prior art are the major reason why only about 15% of voiceless people utilize an artificial larynx. It would be extremely advantageous to humanize the sound emitted by a person using an artificial larynx by improving the tonal quality and reducing the requirements for manual dexterity and coordination so as to produce a less mechanical sounding and more human-like speech.
The present invention avoids all of the problems and disadvantages of the prior art by providing an artificial larynx which produces a more human-like tonal quality and which provides a pitch change circuit wherein the direction of pitch may be manually selected but the total amount of change and the rate of change is pre-established so as to eliminate the users need for manual dexterity and insure the generation of a less monotonous, more human-like quality in the resulting speech.